Carboxylic elastomers



United States Patent 3,403,136 CARBOXYLIC ELASTOMERS John C. Baker, Jr., Dover, Del., assignor to Standard Brands Chemical Industries, Inc., Dover, Del., a corporation of Delaware No Drawing. Filed Sept. 6, 1963, Ser. No. 307,011 11 Claims. (Cl. 260-795) ABSTRACT OF THE DISCLOSURE A carboxylic rubber mix capable of being cured by heating comprising a carboxylic rubbery copolymer and a curing system for preventing precure of said copolymer, said system containing a polyvalent metal curing agent reactable with carboxyl groups of the copolymer, at least about one-fourth of which is present as a peroxide.

The present invention relates to the curing or vulcanization of synthetic rubber compositions, and provides elastic carboxylic polymers having improved cured rubber properties. More particularly, my invention relates to a system for curing polymers having reactive carboxyl groups so that the tendency of such polymers to precure is overcome. At the same time, the invention provides improvement in compression set and ozone resistance of the cured carboxylic rubber and maintains a high level of other vulcanizate properties.

Polymers containing carboxyl (COOH) groups pendant from the polymer chain are polar elastomers characterized by good tensile strength and modulus in the pure gum vulcanizate (unreinforced) state. The carboxyl groups provide sites for cross-linking reactions which cure the polymer and transform it from a plastic to an elastic state. Carboxylic polymers thus possess a means for curing which is not dependent upon the classical sulfur cure or the less common organic peroxide cure systems used for most synthetic rubbers derived from diolefins. Curing of the plastic carboxylic polymers is readily accomplished by reaction of polyvalent metal oxides, hydroxides or weak acid salts with the carboxyl groups in different polymer chains to form elastic metallo-carboxylate rubber polymers. Such carboxylic polymer-s and the elastic metallo-carboxylate polymers resulting from such cross-linking reactions are described in U.S. Patent 2,662,874 to Brown dated Dec. 15, 1953. Carboxylic polymers prepared from diolefins may be cured with metal oxide only, or the metallo-carboxylate cure may be used in conjunction with a conventional sulfur vulcanization as described in the literature Carboxylic Elastomers, Industrial and Engineering Chemistry, vol. 47, #5, pp. 1006 to 1012.

The salt-forming reaction of the carboxylic polymers with polyvalent metals is an ionic type reaction and has a much faster rate than the vulcanization rate with typical sulfur cure systems. This rapid ionic reaction presents a problem because the polymer tends to precure or scorch when the polyvalent metal curing agent is incorporated into the rubber stock. With carboxylic r-ubher the precure problem exists even at room temperature whereas with non-carboxylic rubber stocks scorch generally is not a problem until the rubber mix is heated in processing. Precure or scorch is defined as premature vulcanization, that is, the stock becames partly vulcanized after mixing with the curing system and before it is ready to be vulcanized. Such precure or scorch toughens the rubber and makes it less plastic so that it can no longer be processed or fabricated. Precure or scorch results in scrapped stock or defective products. Thus, carboxylic rubber provides many advantages, but presents a serious or critical precure problem.

I have discovered a curing system for carboxylic rubbery polymers which not only obviates this precure tendency, but also provides improved compression set and ozone resistance in the cured rubber. At the same time the curing system of the invention maintains other vulcanizate properties at desirably high levels.

I have found that supplying a part or all of the polyvalent metal cross-linking agent in the form of peroxide retards the salt-forming metallo-carboxylate reaction and overcomes the tendency of the rubbery polymer to precure or scorch. This improved result and the improvements in cured rubber properties above-mentioned are obtained whether the carboxylic polymer is compounded with a cure system consisting only of the polyvalent metal or also containing sulfur or other curin g agents, such as organic peroxides.

Advantageously, the peroxide is added to the rubber mix as a part or all of the polyvalent metal curing agent and not as a separate and additional component of the rubber batch. In practice, it is preferred to use a mixture of polyvalent metal oxide and peroxide, since commercially available peroxides are often supplied that way. Any of the polyvalent metal oxides or hydroxides (hydrated oxides) set forth in the above-mentioned US. Patent 2,662,874 are suitable as cross-linking agents in the present invention, provided they are used with a peroxide of the same and/or ditferent metal. If the particular polyvalent metal selected does not form a peroxide, then the peroxide of a different polyvalent metal may be used in admixture with the oxide of the other metal. Mixtures of polyvalent metals and their oxides and peroxides provide good results. The peroxides and oxides of metals of Group II of the Periodic Table, and in particular zinc, calcium and magnesium are preferred because of the excellent physical properties developed in the rubber and because of their availability. Zinc peroxide has an advantage when a sulfur cure is used in that it functions as an activator for the accelerator in the sulfur cure system, and also supplies the polyvalent metal for the metallo-carboxylate cross-linking reaction.

It may be characterized as unexpected that the improved results of the invention are obtained with the peroxide present in only minor amount relative to the total amount of polyvalent metal used for the metallocarboxylate curing reaction. For example, as little as one half part of peroxide per hundred of rubber are eifective based on a total polyvalent metal compound charged of one or two parts per hundred of rubber. On the other hand, as stated above a major portion of the polyvalent metal may be in peroxide form, or all of it if the peroxide is available and is sufiiciently stable in pure form. Preferably the metal peroxide should not decompose appreciably until subjected to curing temperatures. Advantageously, the invention provides a control over the rate of the curing reaction by using mixtures of oxides and peroxides and varying the proportions or by selecting polyvalent metals whose peroxides decompose at different temperatures.

The total quantity of polyvalent metal incorporated The rubbery carboxylated polymers for the tests in the rubber mix for the curing reactions will generally set forth below were prepared by aqueous emulsion be not less than about one-half of the stiochiometric or polymerization of the selected monomers to give the theoretical amount based on the carboxyl equivalents in desired synthetic rubbery carboxylic polymer followed the polymer (expressed as equivalents per hundred parts 5 by coagulation of the latex and drying of the polymer. of rubber ephr.) and amounts not substantially exceed- The polyvalent metal oxide and/ or peroxide cross-linking ing twice theoretical are advantageous in developing agent is incorporated by mill mixing or in other suitable optimum cured rubber properties. In general the carboxyl mixer, e.g. Banbury. Other curatives and reinforcing content of the polymer may vary from as little as agents indicated as used in the examples given below about 0.02 ephr. to about 0.5 ephr., or from about 1% to are incorporated in like manner or by known master- 40% acid by weight. batching methods, or other methods conventional in It is to be understood, however, that the invention rubber production which are carried out on ordinary embraces cure systems in which typical sulfur vulcanizarubber mill equipment. tion recipes containing zinc oxide as an activator are used. The physical properties developed in the cured rubber In such case the zinc oxide may be Charg d i 6011- are reported in the tests tabulated below, including comventional amounts e.g. 25 parts per hundred of rubber pression set and ozone resistance (Table X) properties as an activator for the sulfur cure accelerator and will hi h are improved by th u of peroxide i th cure also supply the polyvalent metal for the metallo'carboxsystems f th b li polymgrs, I i 11 known ylate Cross-linking reaction Consequently, the inventhat carboxylic rubbers cured without sulfur or organic tion is not limited a polyvalent metal Charge based on peroxide, that is, only by metallocarboxylate cross-linking the stoiehiolhetry of the reaction of the li y metal reaction have relatively high compression set values. With the eafhoXyl groups in the P ymer. As shown in the tests set forth below the present invention As above stated, in the Production of Vulcanized rubber accomplishes significant improvement in compression set products, it is necessary to avoid Premature vulcanization not only with sulfur cured stocks, but also where the 0f the rubber When miXed With the Curing ingredients carboxylic polymer is cured only with polyvalent metal. Even P Preeufing during mixing and The following example typifies the preparation by Processing must be avoided to allow the Thhhel' mix aqueous emulsion polymerization of carboxylic polymers to he Worked P p y 0n the mill, to he ealehdered into to which the invention pertains and will not be repeated sheet, 9 10 be p in molding, before it is cured or for each polymer, since their synthesis is not a part of Vllleanlledthis invention. The test data on these polymers set forth in The standard test used in the industry for determining h bl ibelow pr vide numerous complete specific the Pfeellre 0f scorch tendency of a rubber mix is referred embodiments of the invention and illustrate but do not to as the Mooney Scorch Test which utilizes a Mooney li i i scope Shearing Disc Viscometer. The test is essentially a meas- Example urement of viscosity increase due to cross-linking of polymer chains which is the basic mechanism of vulcan- A monomer miXtufe 0f 59 Parts hllmdihe, 32 Parts ization. Premature vulcanization occurring in the rubber aerylonitfile and 9 parts methacrylic acid was subjected i i id d b an increase i Mooney i it of to the emulsion polymerization in 125 parts of water using the rubber stock. In the test a shear is exerted on a rubber 4 Parts of a sodium alkylafyl sulfonate as an anionic sample with a small rotor having a disc of 1 inch emulsifying agent, 0.02 part of potassium persulfate as in diameter, with the sample heated to a temperature of initiator, 2 part of a chelating ag ethylehediamihe- 250 F. Increase in shear strength or viscosity is indicated tetraacidic acid, and p of a modifier consisting Of b th diff i d fl ti of a di l gauge, th mixed tertiary C to C mercaptans. The reaction was deflection of which is proportional to the true mean conducted at 50 C. and W218 terminated by addition Of viscosity of the sample. The tendency to scorch is reported 0.5 part hydroquinone as shortstop after approximately as the Mooney Scorch in minutes. This is the measured 75% monomer conversion. 1 part of a stabilizer, phenyl time required for a selected increase in deflection of the beta naphthylamine, was added to the finished latex, dial due to increase in viscosity of the sample. In the which was then coagulated by addition of sodium chlotests reported herein, the Mooney Scorch time is reported ride with addition of ethyl alcohol. The coagulated latex in minutes for a five (5) point increase in deflection, and was well washed and then dried to a crumb in an air oven. in some instances also for a twenty or thirty point The resulting plastic polymer had a carboxyl content of increase in deflection. The Mooney Scorch is measured approximately 0.09 ephr. In the tables below the carboxyl within four hours after mixing the curatives into the content of the polymers is expressed as equivalents per polymer, and also after seven (7) days aging at room hundred of rubber, that is, ephr. -COOH. temperature in order to provide an indication of bin The carboxylated polymers prepared as above exemplistorage stability or shelf life of the mix. fied were mixed with the recipes given in the tables set The invention is more specifically described below in forth below, and the Mooney Scorch test was run on the conjunction with the examples and test data which uncured stocks. In all of the test data set forth below illustrate many variations in the practice of the invenparts are given as parts per hundred of rubber, except tion and the results obtained. where otherwise indicated.

TABLE I Recipe A B C D E F G Carboxylic rubber 100 100 100 100 100 100 SRF Black (Furnex) 40 40 4O 40 40 40 40 Dibutyl Phthalate.

A 1d Altaxbenzothiazyl disulfide ZnO #20 2 Zinc Peroxide 4 Calcium Peroxide Magnesium Peroxide 6 5 MOONEY SCORCH AT 250 F.

Within 4 hours of mixing- 5 point rise Lowest reading- TABLE I-Continued CURED PROPERTIES Cure, Minutes at T.

20' at 307 F. 30 at 307 F. 20 at 307 F. 20' at 307 F. 20 at 307 F. 30 at 325 F. 30' at 325 F.

Tensile, p.S.i 3, 890 2, 920 2, 020 3, 740 3, 840 3, 840 3, 620 Elongation, percent. 410 460 510 460 420 220 310 Hardness Shore 86 78 79 87 93 92 89 Modulus 300%, p.s.i 3,210 2,250 1, 300 3,095 3,310 3,567

Cure, Minutes at T.

30 at 307 F. 40' at 307 F. 60 at 200 F. 60 at 200 F. at 200 F. 60' at 325 F. 60 at 325 F.

Tensile, psi 3, 530 3, 330 300+ 290+ 3, 250 4, 240 3, 946 Elongation, percent. 300 430 1,300+ 1 300+ 430 225 280 Hardness Shore A 87 78 e0 00 92 02 91 Modulus 300%, p.s.i. 3,580 2, 670 152 195 2, 720

1 A butadiene/aciylonitrilelmethacrylie acid polymer from a 59/32/9 00% 02101, balance CaO and Ci1(OH)z. mgnrgng charge; ratio with about 0.09 ephr. COOH. 113% MgO ,'balancs MgO and Mg(01l)z.

u er gra e. 0 rise in 30 minutes. 3 55% Z1102, balance ZnO and Zn(OH)2. 4 Same as Z110 but coated with hydrophobic high molecular weight organic material.

Recipes C, D, E, -F and G show the effect of peroxide 20 TABLE II-Continued where the carboxylic rubber contains only polyvalent Cured Properties metal as curing agent, while recipes A and B show the u Miro r, elfect where a sulfur cure is also used The substantially 30 307 60 325 30 30 0 0 equal Mooney scorch values of stocks A and C show l l I 7 3 ,3 7 60/3 that the precure tendency of these rubber mixes is due 25 g giggs g -f 128 33 '23 32% 31:90 to the rapid ionic cross-linking reaction of the polyvalent Hardness s'hoE-o n s2 s2 s5 s5 85 metal oxide and the carboxyl groups to form the metallo- Modulus 300% 2960 31340 31400 carboxylate, and the precure tendency is not materially Air Oven Agcd70 Hours at 212 F. influenced by the sulfur cure system when present. Stock Cure Min [0 F E shows a very pronounced precure, it being known that 30 calcium gives a faster carboxylate cure than zinc. 30/307 60/325 20/307 29/307 60/325 The presence of peroxide in the B, F and G stocks Tensile, p.s.i 3,900 3,580 4,000 3,840 3.690 controls the reaction rate and eliminates the tendency to gg a ff gg gfgi 3 3; g; 32 precure as shown by the fact that there is no discernable i. H a rise n1 Mooney Scorch in 30 minutes. In addrtion, the 35 Compmssw Set B at 212 lower initial Mooney viscosities Lowest Reading values Cure, Min.l F. of the peroxide-containing stocks show that there is less 45,307 60/305 45/307 45/307 69/325 cross-linking in the polymers at the time of testing. The il values developed in the cured peroxide-containing Pelee Set 4&5 stocks are directly comparable to those obtained in the 40 1 Sallie os z o #20 but coated with hydrophobic high molecularweight organic ma erla g D i E ghgg fil lu gsgg a rfd ggfiie 1:2 2 Values in parentheses are percent loss in elongation on aging.

an e re 1.e. ture is sufiicient to develop adequately the tensile and 313 i g data Tam}; H Zhows i gf other physical properties. For example, with a higher factor 2 ag l a e Sadety charge of the rapid curing calcium hydroxide, 60 minutes h g h 5 e mlxe S 0c w n 2 15 P as at 200 F. was adequate, but a higher temperature was S l t Ooney Scorch Values Wlth nly slight rerequired to develop the tensiles in Stocks B C and D duciton 1n ultimate tenslle strength for both unaged and cured with the zinc peroxide-oxide blend or with zinc aged stocks The Z1102 cured compounds have Oxide slight advantage over the Zn( cures on the basis of per- Stocks F and G also demonstrate that mixed peroxides cent loss. mhelongatlim on agmg' A Y pronounced adof different metals are eflectivc, as well as zinc peroxide. V-anta-ge m t e pe-roxlde cured stocks the parka-d redu- In the tests Shown in Table II a commercial carbox tion in compression set compared to the zinc-oxide com- 3 ylated nitrile rubber, Hycar 1072-A and a butadiene/ l ggfs E VI inclusiv Sho th a bTt f acrylonitrile/methacrylic acid copolymer containing aph e W pp 0 proximately 009 ephr of carboxyl were i ed nventionlto a variety of carboxyhc polymers, filler tested in a sulfur cure recipe with and without zinc acce erators and cure Systems Paroxide TABLE III Receipe A 13 TABLE 11 Carboxylic Rubber 1 100 100 Rccipe A B o D E glg g gg a i 0- 40 40 l u y a a e. 5 5 Nitrile Rubber with 0.00 Steam M 1 1 ephr. ooorr 100 100 .5 H ooi- 1072 (about 0.1 ltaX-m 1 1 ephr. co 100 100 0: (55%)- 5 SRF Black (Fumex) 40 40 40 40 40 #20 5 Dibutyl Phthalnte. 5 a gulfunxngh 9 Mooney Scorch at 250 F.

tea ic ci Alta r Within 4 hours after mixing: ZnO #20.- i i r156 12 A 4320.211 owes rea mg Zinc Peroxide 55 5 After 7 days bin aging at room temp 5 point rise 32 14 Mooney Scorch at 250 F. 7(') Lowest readiiig: 13 16 Within 4 hours of Mixing: 29 50 19 19 4 Unaged Cured Propcrties-Oure 40 at 325 F.

5 point rise Lowest reading 19 19 20 21 20 Te sile, p.s.1 3, 515 3, 730 After 7 Days Bin Storage: 03 43 17 15 27 g olz g lorg leree t. 23g 35(1) fipoint rise at ness ore Lowest reading 19 17 21 21 19 75 Modulus 300%,p.s.i 3,080

TABLE III-Continued Air Oven Aged 70 Hrs. at 212 F.

TABLE V-C011ti11ucd Cured PropertiesCure, 40 M 325 F.

p- 3 860 3 680 Tensile .51 2 820 2 340 3 400 3,015 Elongmilonv Percent '160. '250 Elongatign, pcrccnt 220 I250 Y 270 2.20 Hardness Shore A" 8b 84 r Hardness Shore A 93 91 so 79 1 o O Compresslon Set Method "B' 0 at 212 F- Cum at 325 F- 1 Butadiene/styrene/methacrylic acid copolymer, 63/27/10,0 .10 cphr.

OOH. Average Mooney viscosity, ML (1 4') 212 F. 45. Percent Set A? Butadwienolacrylic acid \cfipolyiner, 90/ 101 0.057 cplir. COOH.

. l t 1 4 212 .=59. Butadlene/acrylonitrilc[sorbic acid copolynier, 55/30/15, 0.05 ephr. vcnge money vlscos! 00011. Average Mooney viscosity, ML (1+4') 212 F.=38. b The Sorblc ig i f ltest 3 29; gjgj ig 0 The results shown above in Table V demonstrate that y fi g emu g ace the scorch resistance with the peroxide-containing stock e e glven 6 p0 ymenza Ion is from 3 to 4 times greater than that of the zinc oxidelPg i g f g g ggg gjg fg Ion er Scorch containing stock when tested within four hours of mixin zi maintainin roxfmatel 6 Hal ed and r The stock aged seven days before test shows a scorch ulmes d ur d rubbe f 55 A g advaIglhl e of resistance about five times that of the stock without zinc z g f elower rgssio is obtainelgwith peroxide. In these recipes a blend of diflerent blacks was if 1 r g co inopsto e used and additional acceleration (Thionex) was provided 6 pa 0 n m 2 55 in the curing system. These results indicate a very fast scorchy cure for the stocks without zinc peroxide. With Recipe. A B the peroxide stocks, however, the much longer scorch carboxync Rubber 100 100 time indicates ability to process these stocks safely both ur 60 60 lmmediately after mlxing and after an appreclable stor- Dibutyl Phthalatc. 5 5 a Stearic Acid.. 1 1 avg McthylTuads. g 3 In Table VI a non-black filler load1ng was employed 2g '8 "I; 5 2D and an organic peroxide vulcanizing agent (DiCup-di- ZnO #20 5 cumyl peroxide).

Mooney Scorch 250 F.

Within 4 hours of mixing: TABLE VI 5 point rise 22 6 20 point rise. 48 15. 5 Recipe A B 0 Lowest reading 22 31 Alter 7 days bin aging at room temp. Carboxylic rubber 100 100 100 5 point rise 21 7. 5 Calcenc T-precipitated CaCOz 40 40 40 20 point rise. 45 22 Hard Clay 40 40 4O Lowest reading 24 40 Cumar P25 (eumarone-idcnc resin) 10 10 10 Stearic Acid 1. 5 1. 5 1. 5 Unaged Cured Propcrtics0ure 40 at 325 F. .i. 1. 15 .5 .5 1. 5 Tensile, p.s.i 3, 120 2, 810 Elongation, per 320 520 5 5 Hardness Shore A". 74 70 Modulus 300%, p.s.i 2,810 2, 075 Mooney Scorch at 250 F.

Air Oven Aged 70 Hrs. at 212 F. Within 4 hours:

5 point rise 13 12 14. 5 Tensile, p.s.i 3, 215 3,105 20 point rise 35 51 58 Elongation, percent" 310 440 Lowest reading 28. 5 29.5 30 Hardness Shore A 75 75 Cured Properties-Cure 55 at 325 F. Compression Set "B 70 Hrs. at 212 F.Curc at 325 F.

Tensile, p.s.i 3,240 2,890 3,090 Percent Set 31. 9 36. 6 Elongation, percent. 320 260 250 45 Hardness Shore A 89 9O 91 Butadiene, acrylonitrile, itaconic acid copolymer 55/30/15, of 0.13 Modulus 300%, p.s.i 2,990 ephr.COOH. Average Mooney viscosity, ML (1+4) 212 F.=45. o

The foregoing test data show that the peroxide pro- Oven hours at 212 From; 40 vides the same advantageous result of much lower Mooney g ensile p.s.i 3,92 4, 200 3,340 onga ion percen 1 120 Scorch with cure systems where sulfur generatmg curmg Hardness h n 94 94 93 agents are subsltuted for elemental sulfur and Where the H q o 0 acid of the polymer is polyicarboxylic (Lg. itaconic At Compression Set B 70 Hrs. at -12 F.Cure 45 Min. at 325 F. the same time, the tensile properties of the unaged and Percent Set 61.2 45.2 40.2 agedcured stocks ar comparable and a s1gn1ficant reduc- Same as Table II recipes C and tion in compress1on set 1s obtamed with the peroX1de-c0ntaining rubber mix.

TABLE v These tests indicate that with an organic peroxide type Recipe A D C D cure, the zinc peroxide presents a definite advantage in C b r bb l 100 100 scorch resistance, part1cularly at the 20 point rise level. 02510215115 5315,35 21:31:11: {its "165 A180, vylth t dlfferenttype ofpure ys p q ed g fl gg fi g 3g 3% reduction 1n compression set is obtained ust as 1n the ,3,5; u; l jjjj 20 20 case of sulfur type cures and the straight metallo-carbox- Circolite Oil (Petroleum rubber process ylate cures,

oil) 7. 5 7. 5 10 10 Stcaric 1. 5 1. 5 1. 5 1. 5 SulfuL--- 2 2 2 2 Altax 1.5 1.5 1.75 1. 60 TABLE VII Thionex (secondary accelerator). 15 15 20 20 n 0 5 5 Recipe A B C D E Z1102, 55% 5 5 Carboxylic Rubber 1 100 100 100 100 Mooney Scorch at 250 F. Furnex SRF Black 40 40 40 40 40 Dibutyl Phthalate. 5 Within 4 hours of mixing: 7() Stearic Acid 5 point rise 6. 5 21 4 14 Su1fur 20 point rise 9. 5 36 5 5 22 Alta r." Lowest reading 14 13 30 25 Z #30. After 7 days bin aging at room temp. ZnO #20-2L. 5 point rise 5. 5 25 2. 5 13 Z1102, 28% 20 point rise 7. 5 34 4. 5 20. 5 Z1102, 55% Lowest reading 13 12 41 25 Z1101, 70%

Within 4 hours:

5 point rise 18. 5 17. 5 27 31 34 point rise 24. 5 24 37 44. 5 46 20 point rise 33 32 54 72 71 Lowest reading 27 27 25 26 27 v After 7 days bin aging at room temp.:

5 point rise 14. 5 25 29 32 10 point rise 19 19. 5 34 41 43. 5

20 point rise 25 46 63. 5 63. 5

Lowest reading 28 28 25 26 27 After days bin aging at room temp.:

5 point rise 12 25 27 29 10 point rise 15. 5 32. 5 39. 5

20 point rise 20 41. 5 60 60 Lowest reading 27 28 24 25 2S Cured Properties Cure, Min./ F.

Tensile, p.s.i. 4, 260 4, 105 4, 210 3, 840 3, 740 Elongation, percent 310 310 270 300 330 Hardness Shore A 89 89 88 86 86 Modulus 300%, p.s.i 4,160 4,000 3,840 3,585

Oven Aged-70 Hrs. at 212 F-Cure 40 Min.

4, 000 4, 200 4, 375 4, 150 4, 055 Elongation, perce 180 210 170 200 220 Hardness Shore A 92 93 93 92 92 Compression Set B 70 Hrs. at 212 F-Cure Min.

Percent Set 49. s 48.8 as. 2 33.8 as. 7

1 Butadienelacrylonitrilelmethacrylic acid, 0.09 eplir. --COOH. 2 Balance ZnO and Zl1(OH)2.

The foregoing table records results of tests on recipes having varying quantities of peroxide and compares such recipes with those without peroxide. Significantly higher Mooney Scorch resistance, and longer Mooney Scorch time, are obtained with relatively small amounts of peroxide. The 28% zinc peroxide-zinc oxide blend provides only 1.4 parts zinc peroxide per hundred parts or rubber. Increasing the quantity of zinc peroxide provides an ad ditional measure of scorch resistance, but the difference between the ZnO blend and the 70% ZIllO'g blend is relatively small. These data indicate the peroxide may be only a minor fraction of the total polyvalent metal charge. A

The compression set properties were fairly comparable throughout the entire range of peroxide content in the foregoing test recipes and are greatly improved over the stocks containing no peroxide.

In Table VIII below. are shown test results in which the quantities of zinc oxide and zinc peroxide were widely varied while still obtaining the benefits of the invention.

TABLE VIII Recipe A B C D E Carboxylic Rubber 100 100 100 100 100 Furnex (SRF Black). 40 40 40 4O 40 Dibutyl Phthalate 5 5 5 5 5 Stearic Acid 1 1 1 1 Sulfur 1. 5 1. 5 Altax 1 1 ZnO #20 2 3.5 Z1102, 55%... 1.5 1.5 Kadox 72 2 Mooney Scorch at 250 F.

Within 4 hours:

5 point rise 26 21 16 11 27 30 point rise 44 34 30 Low reading 19 20 20 23 20 Compression Set Method B 70 Hours at 212 F.Cure 45' at 325 F.

Percent Set 40. 2 36. 7 45. 7 99. 1 86. 6

C6%?adiene/acrylonitrile/methacrylic acid copolymer 0.09 ephr.-

2 Rubber grade ZnO-New Jersey zinc.

3 14 pts. in 90'.

In the foregoing tests the peroxide content varied from about 0.8 up to over 4 parts. The total polyvalent metal charge varied from about 3.5 to 7.5 parts, which based on the carboxyl content of the polymer represents a variation of from just under stoichiometric amount to over twice theoretical.

As may be seen from the Mooney Scorch values, the peroxide substantially reduces the scorch tendency in all cases, whether the cure system consists solely of polyvalent metal cross-linking agent or also includes sulfur cure system. It should be noted that the Mooney Scorch value of stock C compounded with the sulfur cure system is comparable to that of recipe F containing only polyvalent metal. This clearly indicates that the scorchiness or precure tendency in these stocks is due to the rapid ionic reaction between the polyvalent metal and the carboxyl groups andcannot be attributed to the slower reactions of the sulfur cure.

The data presented in Table IX below illustrate application of the invention to other polyvalent metals in straight metallo-carboxylate cures without sulfur.

TABLE IX Recipe A B O D E F Carboxylic Rubber 1 100 100 100 100 100 Furnex SRF (Black) 40 40 40 40 40 40 Dibutyl Phthalate.-- 5 5 5 5 5 Stearic Acid After 7 days at room te 25321535331211 ..3 12 it 13 i3 2: 33 Low reading 19 16 21 19' 25 17 Cured Properties Compression Set B 70/212-(Cure 60/307) Percent Set 100 68. 3 105. 2 87. 1 100. 4 97. 9

' 13" Fuel 70/RT--(Cure 60/307) Volume Change 30.8 29.3 29.0 26. 6 27.9 30.4

1 Butadiene/acrylonitrile/methacrylic acid copolyrncr, 0.09 ephr.

OOH. 2 Balance ZnO+Zn(OH)2. 3 Balance CaO+Ca(OH)z. 4 Balance MgO+Mg(OH)z.

-In stocks D and F the calcium and magnesium peroxides provide Mooney Scorch values which are reduced by a factor of two to four fold compared to stocks C and E without peroxide. In addition, although the compression set values of these stocks cured without sulfur are high, in each case those stocks cured with the peroxide exhibit significant improvement in compression set.

The presence of peroxide in the carboxylic polymer provides a further advantage in the improved ozone resistance of the cured stock. This is shown in the Table X below illustrating outdoor aging tests using samples with triangular shaped cross section, the tests being conducted as prescribed in ASTM Aging Test D 1171-61. Recipes A and B of Table IX above were used in these tests with variations in the stock shown below in the table. The compounded rubber was cured for sixty minutes at 307 F. and then after cool down was subjected to the outdoor aging test with the results as recorded below.

TABLE X [Outdoor Aging ASTM Test Method D 1171-61] Rating and Date (1963) Recipe Recipe A of Table IX (ZnO) 0 0 0 1 3 Recipe A of Table IX (ZnO +3 parts Antichek Wax) 0 0 1 3 Recipe B of Table IX (ZIlOz) 0 0 0 0 2 stock containing the zinc peroxide showed no evidence of ozone cracking. It was not until 8 to 10 weeks exposure that the zinc peroxide-containing stock showed signs of initial ozone cracking and by that time the other test stocks were badly cracked.

Although the curing temperatures in the tests tabulated above are generally 300 F. or higher, it is to be understood that the metallo-carboxylate cure may be conducted at much lower temperatures if sufiicient time is allowed for development of a proper cure. Curing temperatures of from 125 F. to 400 F. may be used.

The invention is applicable to any carboxylated polymer which is vulcanizable or curable with polyvalent metal oxide, including carboxylated natural rubber. Such polymers include those mentioned in U.S. Patent 2,662,- 874 all of which are copolymers of aliphatic conjugated dienes such as butadiene-1,3 and derivatives, preferably having 4 to 6 carbon atoms. In addition to aqueous emulsion polymerization, such carboxylated copolymers may be made by the other methods such as disclosed in the above-mentioned patent. Of course, where it is desired, the carboxylated rubber may be blended with noncarboxylic stocks in which case sulfur or organic peroxide curatives must be used.

The invention is furthermore fully applicable to carboxyl-containing acrylate polymers and other carboxylated polymers made from monoethylenically unsaturated monomers only, where such polymers are curable to elastomeric form by reaction of polyvalent metal crosslinking agents with the carboxyl groups.

For the majority of applications of the dry rubber compounds to which the present invention pertains, carboxylated butadiene copolymers are preferred in which the diene content of the polymer is at least about 40%,

preferably at least 50% for developing the most desirable elastomeric properties.

Any of the monoethylenically unsaturated monomers copolymerizable with butadiene may be used for the preparation of ter and multipolymers of the carboxylic type. Alkenyl aromatic monomers, such as styrene and aliphatic nitriles such as acrylonitrile are preferred comonomers, although any others such as vinyl pyridine and acrylic acid esters may be mentioned as suitable. When such cmonorners are used, they may be present in widely varying ranges from about 1% up to about 50% by weight.

The invention, as shown by the foregoing description and test results, contributes an improved cure system for carboxylic rubber which overcomes the precure or scorch tendencies of these polymers when mixed with polyvalent metal curatives. This insures safer processing before cure and avoids scrapping of stock or production of defective products. At the same time the cured stock physical properties are enhanced or maintained at high levels.

Numerous modifications and additions to the specific embodiments illustrated above will be apparent to those skilled in the art. Such changes may be made without departing from the scope of the invention which is to be determined by the appended claims.

I claim:

1. A carboxylic rubber mix capable of being cured by heating comprising a carboxylic rubbery copolymer and a curing system for preventing precure of said copolymer prior to curing by heat, said system containing a polyvalent metal curing agent reactable with carboxyl groups of the copolymer, at least one-fourth of which is present as a peroxide.

2. A rubber mix according to claim 1 in which said curing system also contains a curing agent other than said polyvalent metal curing agent.

3. A rubber mix according to claim 2 in which the other 12 curing agent is sulfur and the'curing system also contains an organic cure accelerator.

4. A rubber mix according to claim 1 in which the other curing agent is an organic peroxide.

5. A rubber mix according to claim 1 in which the polyvalent metal portion of the curing agent is a metal of Group II of the Periodic Table.

6. A rubber mix according to claim 1 in which the metal present as the peroxide is selected from the group consisting of calcium, magnesium, zinc and mixtures thereof.

7. A rubber mix according to claim 6 in which said copolymer is a diene copolymer having a carboxyl (COOH) content of from about .02 to about 0.5 ephr. and said curing system contains at least about 0.5 part of the polyvalent metal peroxide per parts of the copolymer based on 1 to 2 parts of the polyvalent metal curing agent per 100 parts of the rubbery copolymer.

8. A rubber mix according to claim 6 in which said copolymer is a diene copolymer having a diene content of at least about 40% by weight and a carboxyl (COOH) content of from about .02 to about 0.5 ephr. and said curing system contains a polyvalent metal curing agent, at least about one-fourth of which is zinc peroxide, in an amount not less than about one-half and not substantially in excess of twice the stoichiometric amount with respect to the carboxyl (COOH) group content of the copolymer.

9. A carboxylic rubber having improved scorch resistance and improved compression set characteristics when cured comprising a rubbery copolymer containing at least about 40% by weight of a polymerized aliphatic conjugated diolefin, from about 1 to about 50% by weight of a copolymerized monoethylenically unsaturated monomer and the balance being up to about 40% by weight of a copolymerized :aliphatically unsaturated carboxylic acid, said copolymer having a carboxyl (COOH) group content of from about .02 to about 0.5 ephr. and being cured by heating to a temperature of from about F. to about 400 F. with a curing system containing a poly valent metal curing agent in an amount not less than about one-half and not substantially exceeding twice the stoichiometric amount with respect to the carboxyl (COOH) group content of the copolymer and at least about onefourth of the polyvalent metal curing agent being present as a peroxide, said copolymer exhibiting a substantially reduced compression set compared to polymers cured by said system in the absence of said polyvalent metal peroxide.

10. A carboxylic rubber according to claim 9 in which the polyvalent metal present as a peroxide is selected from the group consisting of calcium, magnesium, zinc and mixtures thereof.

11. The carboxylic rubber according to claim 9 in which the curing system also contains a polyvalent metal curing agent present in the form of a compound selected from the group consisting of oxides, hydroxides and mixtures thereof.

References Cited UNITED STATES PATENTS 2,669,550 2/1954 Brown 260-80.7 2,724,707 11/1955 Brown 260-807 2,849,426 8/1958 Miller 260-807 3,093,620 6/1963 Gladding.

OTHER REFERENCES Zapp et al., J. Pol. Sci., 9, 97-413 (1952).

JOSEPH L. SCHOFER, Primary Examiner.

J. C. HAIGHT, Assistant Examiner.

U.S. DEPARTMENT OF COMMERCE PATENT OFFICE Washington, 0.0. 20231 UNITED STATES P k TENT ()FFICE CERTIFICATE OF CORRECTION Patent No. 3,403,136 September 24, 1968 John C. Baker, Jr.

It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as show below:

Column 3, line 3, "stiochiometric" should read stoichiometric Column 8, TABLE V-Continued, in the heading, line 1 thereof, "40M 325 F." should read 40 at 325 F. same column 8 TABLE VI second column, line 28 thereof, "61 2" should read 61.6 Column 9, TABLE VIII, first column, line 1 thereof, "Carboxylic Rubber" should read Carboxylic Rubber Signed and sealed this 17th day of February 1970.

(SEAL) Attest:

Edward M. Fletcher, Jr. E. Attesting Officer Commissioner of Patents 

